1. TECHNICAL FIELD
The present invention relates generally to microcomputers and processes of producing such microcomputers. More particularly, the present invention relates to an ASIC (IC for specific use) type microcomputer for exerting control by reference to a data table (data in the form of a table), the microcomputer being capable of readily setting the position of the data table set on ROM to the rearmost part of the storage area thereof, and a process of producing the same. Further, the present invention relates to a microcomputer capable of having ROM with various storage capacities packaged without changing the contents of microprograms and instructions on reference to the data table, and a process of producing the same.
2. Background Art
Microcomputers in the form of one chip are widely used in control circuits, remote controllers and the like of cameras, home electric appliances and many other kinds of electronic equipment, these being generally under 4- and 8-bit control.
A one-chip microcomputer of the sort described above is slightly different in internal construction from a general purpose microprocessor. The one-chip microcomputer has a number of registers, memories, ALU and the like that have been built in one chip and connected via buses. In this case, an I/O buffer connected to an internal bus is used for exchanging signals with an external device. In other words, such a one-chip microcomputer is designed to exchange data with an external circuit via the I/O buffer by having a controller control circuits including registers, memories, ALU and the like, the controller being CPU, a designated gate circuit, a logic circuit or the like.
In the case of the ASIC microcomputer under the control of a microprogram excluding a full-custom case, the most of control circuits are used commonly as hardware. A microcomputer of this sort is designed to suit ASIC as per specification designated mainly by changing the contents of instructions and constant data. In this case, however, the size of a memory such as ROM for storing instructions and constant data often varies, depending on the specification. Moreover, microprograms as programs to be written to ROM are normally unitized according to the processing function; program units for common use and standardized program units among them are stored preferentially and successively. Subsequently, a necessary data table corresponding to a difference in specification and the subject matter of control is packaged in the rearmost storage area of ROM.
When the storage capacity of ROM (ROM size) for storing a microprogram differs, however, the last address also differs depending on the size. Consequently, the position of the rearmost storage area after a data table is arranged varies with the ROM size. When the size of ROM to be packaged differs, an access address should be altered in proportion to the size of ROM to be packaged or the hardware should be modified with respect to the microprogram referred to by the data table, reference instructions of the data table or the like.
However, alteration of the contents of a program or an instruction affects various other programs. As a result, use may be made of a method of dispensing with such alteration. Hardware is changed in accordance with the ROM size in the preceding case. Otherwise, a special program for converting the access address of the data table in accordance with the ROM size is provided to deal with that case programmatically. Notwithstanding, the storage area of ROM is consumed in the latter case to the extent that the programmatic processing is carried out and moreover the problem is that the sufficient storage area of the data table will not be secured.